6man Working Group                                          A. Matsumoto
Internet-Draft                                               T. Fujisaki
Intended status: Standards Track                                     NTT
Expires: May 18, July 20, 2013                                          T. Chown
                                               University of Southampton
                                                       November 14, 2012
                                                        January 16, 2013

           Distributing Address Selection Policy using DHCPv6


   RFC 6724 defines default address selection mechanisms for IPv6 that
   allow nodes to select an appropriate address when faced with multiple
   source and/or destination addresses to choose between.  The RFC 6724
   allowed for the future definition of methods to administratively
   configure the address selection policy information.  This document
   defines a new DHCPv6 option for such configuration, allowing a site
   administrator to distribute address selection policy overriding the
   default address selection parameters and policy table, and thus
   control the address selection behavior of nodes in their site.

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1.  Introduction

   RFC 3484 [RFC3484] describes default algorithms for selecting an
   address when a node has multiple destination and/or source addresses
   to choose from by using an address selection policy.  In Section 2 of
   RFC 6724, it is suggested that the default policy table may be
   administratively configured to suit the specific needs of a site.
   This specification defines a new DHCPv6 option for such

   Some problems have been identified with the default RFC 3484 address
   selection policy [RFC5220].  It is unlikely that any default policy
   will suit all scenarios, and thus mechanisms to control the source
   address selection policy will be necessary.  Requirements for those
   mechanisms are described in [RFC5221], while solutions are discussed
   in [I-D.ietf-6man-addr-select-sol] and
   [I-D.ietf-6man-addr-select-considerations].  Those documents have
   helped shape the improvements in the default address selection
   algorithm [RFC6724] as well as the DHCPv6 option defined in this

1.1.  Conventions Used in This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

1.2.  Terminology

   This document uses the terminology defined in [RFC2460] and the
   DHCPv6 specification defined in [RFC3315]

2.  Address Selection options

   The Address Selection option provides the address selection policy
   table, and some other configuration parameters.

   An Address Selection option contains zero or more policy table
   options.  Multiple Policy Table options in an Address Selection
   option constitute a single policy table.

   The format of the Address Selection option is given below.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      |          OPTION_ADDRSEL       |         option-len            |
      |  Reserved |A|P|                                               |
      +-+-+-+-+-+-+-+-+     POLICY TABLE OPTIONS                      |
      |                      (variable length)                        |
      |                                                               |

                 Figure 1: Address Selection option format

   option-code:  OPTION_ADDRSEL (TBD).

   option-len:  The total length of the Reserved field, A, P flags, and

   Reserved:  Reserved field.  Server MUST set this value to zero and
        client MUST ignore its content.

   A:   Automatic Row Addition flag.  This flag toggles the Automatic
        Row Addition flag at client hosts, which is described in the
        section 2.1 in RFC 6724 [RFC6724].  If this flag is set to 1, it
        does not change client host behavior, that is, a client MAY
        automatically add additional site-specific rows to the policy
        table.  If set to 0, the Automatic Row Addition flag is
        disabled, and a client SHOULD NOT automatically add rows to the
        policy table.

   P:   Privacy Preference flag.  This flag toggles the Privacy
        Preference flag at client hosts, which is described in the
        section 5 in RFC 6724 [RFC6724].  If this flag is set to 1, it
        does not change client host behavior, that is, a client will
        prefer temporary addresses.  If set to 0, the Privacy Preference
        flag is disabled, and a client will prefer public addresses.

   POLICY TABLE OPTIONS:  Zero or more Address Selection Policy Table
        options described below.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      |     OPTION_ADDRSEL_TABLE      |         option-len            |
      |    label      |  precedence   |   prefix-len  |               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               |
      |                                                               |
      |                   prefix   (variable length)                  |
      |                                                               |

            Figure 2: Address Selection Policy Table option format

   option-code:  OPTION_ADDRSEL_TABLE (TBD).

   option-len:  The total length of the label field, precedence field,
        prefix-len field, and prefix field.

   label:  An 8-bit unsigned integer; this value is for correlation of
        source address prefixes and destination address prefixes.

   precedence:  An 8-bit unsigned integer; this value is used for
        sorting destination addresses.

   prefix-len:  An 8-bit unsigned integer; the number of leading bits in
        the prefix that are valid.  The value ranges from 0 to 128.

   prefix:  A variable-length field containing an IP address or the
        prefix of an IP address.  An IPv4-mapped address [RFC4291] must
        be used to represent an IPv4 address as a prefix value.  The
        prefix should be left aligned, big-endian, and zero padded on
        the right up to the next octet boundary.  So the length of this
        field should be between 0 and 16 bytes.

3.  Appearance of the Address Selection options

   The Address Selection options MUST NOT appear in any DHCPv6 messages
   other than the following ones: Solicit, Advertise, Request, Renew,
   Rebind, Reconfigure, Information-Request, and Reply.

4.  Processing the Policy Table option

   This section describes how to process received Policy Table option at
   the DHCPv6 client.

   This option's concept is to serve as a hint for a node about how to
   behave in the network.  So, basically, it should be up to the node's
   administrator how to deal with the received policy information in the
   way described below.

4.1.  Handling of the local policy table

   RFC 6724 defines the default policy table.  Also, users are usually
   able to configure the policy table to satisfy their own requirements.

   The client implementation SHOULD provide the following choices to the

   a) replace the existing active policy table with the DHCPv6
      distributed policy table.
   b) preserve the existing active policy table, whether this be the
      default policy table, or user configured policy.

4.2.  Handling of the stale policy table

   When the information from the DHCP server goes stale, the policy
   received form the DHCP server should be deprecated.

   The received information can be considered stale in several cases,
   such as, when the interface goes down, the DHCP server does not
   respond for a certain amount of time, and the Information Refresh
   Time is expired.

4.3.  Multi-interface situation

   The policy table, and other parameters specified in this document are
   node-global information by their nature.  One reason being that the
   outbound interface is usually chosen after destination address
   selection.  So, a host cannot make use of multiple address selection
   policies even if they are stored per interface.

   Even if the received policy from one source is merged with one from
   another source, the effect of both policy are more or less changed.
   The policy table is defined as a whole, so the slightest addition/
   deletion from the policy table brings a change in semantics of the

   It also should be noted that absence of the distributed policy from a
   certain network interface should not be treated as absence of policy
   itself, because it may mean preference for the default address
   selection policy.

   Under the above assumptions, how to handle received policy is
   specified below.

   A node MAY use Address Selection options by default in any of the
   following two cases:

   1: The host is single-homed, where the host belongs to one
      administrative network domain exclusively usually through one
      active network interface.
   2: The host implements some advanced heuristics to deal with multiple
      received policy, which is outside the scope of this document.

   The above restrictions do not preclude implementations from providing
   configuration options to enable this option on a certain network

   Nor, they do not preclude implementations from storing distributed
   address selection policies per interface.  They can be used
   effectively on such implementations that adopt per-application
   interface selection.

5.  Implementation Considerations

   o  The value 'label' is passed as an unsigned integer, but there is
      no special meaning for the value, that is whether it is a large or
      small number.  It is used to select a preferred source address
      prefix corresponding to a destination address prefix by matching
      the same label value within the DHCP message.  DHCPv6 clients
      SHOULD convert this label to a representation appropriate for the
      local implementation (e.g., string).

   o  Currently, the label and precedence values are defined as 8-bit
      unsigned integers.  In almost all cases, this value will be

   o  The maximum number of address selection rules that may be conveyed
      in one DHCPv6 message depends on the prefix length of each rule
      and the maximum DHCPv6 message size defined in RFC 3315.  It is
      possible to carry over 3,000 rules in one DHCPv6 message (maximum
      UDP message size).  However, it should not be expected that DHCP
      clients, servers and relay agents can handle UDP fragmentation.
      Network adiministrators SHOULD consider local limitations to the
      maximum DHCPv6 message size that can be reliably transported via
      their specific local infrastructure to end nodes; and therefore
      they SHOULD consider the number of options, the total size of the
      options, and the resulting DHCPv6 message size, when defining
      their Policy Table.

   o  Since the number of selection rules could be large, an
      administrator configuring the policy to be distributed should
      consider the resulting DHCPv6 message size.

6.  Security Considerations

   A rogue DHCPv6 server could issue bogus address selection policies to
   a client.  This might lead to incorrect address selection by the
   client, and the affected packets might be blocked at an outgoing ISP
   because of ingress filtering, incur additional network charges, or be
   misdirected to an attacker's machine.  Alternatively, an IPv6
   transition mechanism might be preferred over native IPv6, even if it
   is available.  To guard against such attacks, a legitimate DHCPv6
   server should communicate through a secure, trusted channel, such as
   a channel protected by IPsec, SEND and DHCP authentication, as
   described in section 21 of RFC 3315,

   Another threat is about privacy concern.  As in the security
   consideration section of RFC 6724, at least a part of, the address
   selection policy stored in a host can be leaked by a packet from a
   remote host.  This issue will not be modified by the introduction of
   this option, regardless of whether the host is multihomed or not.

7.  IANA Considerations

   IANA is requested to assign option codes to OPTION_ADDRSEL and
   OPTION_ADDRSEL_TABLE from the option-code space as defined in section
   "DHCPv6 Options" of RFC 3315.

8.  References
8.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3315]  Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
              and M. Carney, "Dynamic Host Configuration Protocol for
              IPv6 (DHCPv6)", RFC 3315, July 2003.

   [RFC3484]  Draves, R., "Default Address Selection for Internet
              Protocol version 6 (IPv6)", RFC 3484, February 2003.

   [RFC6724]  Thaler, D., Draves, R., Matsumoto, A., and T. Chown,
              "Default Address Selection for Internet Protocol Version 6
              (IPv6)", RFC 6724, September 2012.

8.2.  Informative References

              Chown, T. and A. Matsumoto, "Considerations for IPv6
              Address Selection Policy Changes",
              draft-ietf-6man-addr-select-considerations-04 (work in
              progress), October 2011.

              Matsumoto, A., Fujisaki, T., and R. Hiromi, "Solution
              approaches for address-selection problems",
              draft-ietf-6man-addr-select-sol-03 (work in progress),
              March 2010.

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, December 1998.

   [RFC3493]  Gilligan, R., Thomson, S., Bound, J., McCann, J., and W.
              Stevens, "Basic Socket Interface Extensions for IPv6",
              RFC 3493, February 2003.

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, February 2006.

   [RFC4941]  Narten, T., Draves, R., and S. Krishnan, "Privacy
              Extensions for Stateless Address Autoconfiguration in
              IPv6", RFC 4941, September 2007.

   [RFC5220]  Matsumoto, A., Fujisaki, T., Hiromi, R., and K. Kanayama,
              "Problem Statement for Default Address Selection in Multi-
              Prefix Environments: Operational Issues of RFC 3484
              Default Rules", RFC 5220, July 2008.

   [RFC5221]  Matsumoto, A., Fujisaki, T., Hiromi, R., and K. Kanayama,
              "Requirements for Address Selection Mechanisms", RFC 5221,
              July 2008.

Appendix A.  Acknowledgements

   Authors would like to thank to Dave Thaler, Pekka Savola, Remi Denis-
   Courmont, Francois-Xavier Le Bail, Ole Troan, Bob Hinden, Dmitry
   Anipko, Ray Hunter, Rui Paulo, Brian E Carpenter, Tom Petch, and the
   members of 6man's address selection design team for their invaluable
   contributions to this document.

Appendix B.  Past Discussion

   o  The 'zone index' value is used to specify a particular zone for
      scoped addresses.  This can be used effectively to control address
      selection in the site scope (e.g., to tell a node to use a
      specified source address corresponding to a site-scoped multicast
      address).  However, in some cases such as a link-local scope
      address, the value specifying one zone is only meaningful locally
      within that node.  There might be some cases where the
      administrator knows which clients are on the network and wants
      specific interfaces to be used though.  However, in general case,
      it is really rare case, and the field was removed.

Authors' Addresses

   Arifumi Matsumoto
   NTT NT Lab
   3-9-11 Midori-Cho
   Musashino-shi, Tokyo  180-8585

   Phone: +81 422 59 3334
   Email: arifumi@nttv6.net
   Tomohiro Fujisaki
   NTT NT Lab
   3-9-11 Midori-Cho
   Musashino-shi, Tokyo  180-8585

   Phone: +81 422 59 7351
   Email: fujisaki@nttv6.net

   Tim Chown
   University of Southampton
   Southampton, Hampshire  SO17 1BJ
   United Kingdom

   Email: tjc@ecs.soton.ac.uk